A fixed length packet (hereinafter referred to as a cell) transfer in an ATM network has been described in, for example, `Data Communication Using ATM: Architecture, Protocols, and Resource Management,` IEEE Communication Maggin August 1994, pp. 24-31, `SVC Signaling: Calling All Nodes` DATA COMMUNICATIONS JUNE 1995, pp. 123-128 and so forth.
In an ATM network, a call (connection) is set along a communication channel extending from a sending-side device (calling terminal) as the transfer path of a user cell via a switching device (switch) up to a receiving-side device (called or destination terminal), depending on the signaling process at the time of calling. The cell transfer is controlled on the basis of the connection identifying information attached to the header of each user cell.
A call setting procedure has been mentioned in, for example, ITU-T Standards Q.2931 and by performing the call setting procedure, connection information is set in the sending-side device, each node (switch) on the communication channel and the receiving-side device. The communication channel includes an identifier for identifying a call on each of the links between the sending side and the switch between the terminals, between the switch and the receiving side, a traffic class indicating the cell transfer priority among the switches and so forth. The identifier for identifying the connection (call) is called a VPI (Virtual Pass Identifier) and a VCI (Virtual Connection Identifier), which are set as address information in the header of each cell.
Connection information necessary for the switching processing according to the VPI, VCI of each input cell received through a transmission line is retrieved at each switch. The connection information includes internal routing information (output port number), an identifier (output VPI/VCI) to be attached to an output cell, a traffic class showing the cell priority within the switch and so forth.
The traffic class indicative of the cell priority has been described in, for example, `Multimedia Traffic Management Principles for Guaranteed ATM Network Performance` IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS VOL. 8, NO. 3, APRIL 1990, pp. 437-446 and `Traffic Management for B-ISDN Services` IEEE Network, September 1992, pp. 10-19.
There are two traffic classes, for example, showing cell priority: CBR (Constant Bit Rate) and VBR (Variable Bit Rate). The CBR is a traffic class for ensuring that on the basis of a contract between the network and the terminal for a predetermined cell transfer rate at the time of the setting of a call, the network side insures the cell transfer at the aforementioned transfer rate, whereas the VBR is a traffic class for allowing the occurrence of a statistical swing to a certain degree concerning the transfer rate contracted with the terminal. A traffic control method is called `Preventive Control, and is based on the contract placed between the network and the terminal.
There are also a group of traffic classes called `Best Effort Control` which perform transmission by utilizing the remainder of the band allocated to other terminals at the aforementioned CBR, VBR without any special contract concerning the transfer rate between the network and the terminal at the time of setting a call. One of the reasons for the transfer rate contract not to be held is that the terminal for outputting burst traffic is hardly able to predict traffic characteristics at the time of setting a call.
In the group of Best Effort Control traffic classes are an UBR (Unspecified Bit Rate) traffic class in which the network assures no transfer and an ABR (Available Bit Rate) traffic class which assures the generation of no cell loss by effecting feedback control during congestion between the network and the terminal. Incidentally, the ABR traffic class has been described in, for example, `The Rated-Based Flow Control Framework for the Available Bit Rate ATM Service` IEEE Network March/April 1995, pp. 25-39.
With respect to a switch arrangement for effecting transfer control in accordance with the traffic class, Japanese Patent Laid-Open No. 197128/1994 (prior art 1), for example, describes a packet switching device wherein two output buffers for CBR and VBR classes are provided at each output port so as to store table information representing empty/filled state of the two buffers corresponding to the output port, so that by referring to the table information, an input buffer control unit determines a storage buffer of a cell to be sent to each output port. In this case, the output priority of the cell stored in the CBR buffer is set higher than that of the cell stored in the VBR buffer, whereby communication delay in the switch can be suppressed to a range of predetermined values with respect to the cell group of the CBR traffic whose communication delay is under severe restriction.
In cases where the CBR buffer is not empty, for example, cells are accumulated in the VBR buffer on condition that space exists in the VBR buffer, so that the band within the switch may be efficiently utilized effectively be utilizable. When the ABR, VBR traffic classes are supported, an output buffer corresponding to another traffic class in addition to the CBR, VBR traffic classes may be added.
The technique proposed in `Development of 622 Mbps 8'8 ATM Switch LSI Having 5-Class Delay Priority Control Function`, at the 1996 National Convention B-598, the Institute of Electronics, Information and Communication Engineers of Japan (prior art 2) provides counter information of the number-of-cells for each connection and threshold information for each connection within the same traffic class, with respect to each traffic class of CBR, VBR, are stored in a switch. When the value of the number-of-cells counted exceeds the threshold, cell discard is carried out.
Further, the technique proposed in `Selective Cell Discard Method by Counter Control` at the 1996 National Convention B-765, the Institute of Electronics, Information and Communication Engineers of Japan (prior art 3), for example, proposes that a delimiter in a host protocol packet (information unit handled under the upper-order protocol and comprising a plurality of cells) is recognized and when congestion occurs, selective continuous cell discard otherwise known as selective cell discard, is carried out in units of a packet.
Regarding a switching system configuration for dealing with ATM cells, Japanese Patent Laid-Open No. 276943/1992 (prior art 4) describes providing a common cell storage buffer with respect to a plurality of output ports instead of providing a physically independent buffer for each output port.